Field of the Invention
Embodiments of the present disclosure relate to the art of a shift control system for a vehicle having an automatic transmission.
Discussion of the Related Art
JP-A-2007-138901 describes a control system of an automatic transmission for vehicle that establishes a plurality of gear stages of different speed ratios by selectively engaging a plurality of frictional engagement devices, and that, after increasing/decreasing a drive torque transmitted from an engine to an input shaft, outputs the increased/decreased drive torque. In this kind of shift control system, an inertia torque of a prime mover accompanying a change in speed ratio is added to a drive torque outputted from the prime mover, at a time of a shift due to an accelerator pedal being depressed. As a result, a shift shock occurs due to a sudden change in an output torque of the automatic transmission. Accordingly, in order to prevent such disadvantage, the shift control system taught by JP-A-2007-138901 is configured to execute an ignition retard to retard an ignition timing of the engine in an inertia phase lowering a turbine speed to a synchronous speed in a gear stage after the shift.
JP-A-2013-154723 and JP-A-2011-218835 also describe control systems configured to suppress a shift shock by causing a clutch disengaged during a shift to slip.
JP-A-2010-159815 describes a control system configured to suppress a shift shock by causing a lockup clutch of a torque converter to slip during a shift.
JP-A-2005-313786 describes a control system for a geared transmission configured to cause a lockup clutch, disposed between an engine and an automatic transmission, to slip or release the lockup clutch when a rotational speed of an input shaft of the transmission is substantially synchronized with a synchronous speed of a gear stage to be established after a shifting operation.
According to the teachings of JP-A-2007-138901, the ignition retard is executed to reduce fluctuations in output shaft torque. However, there is a limit to an ignition retard amount, and sometimes, due to an operating state, fluctuations in output shaft torque cannot be sufficiently suppressed. Moreover, sometimes, due to lowering of engine output caused by ignition retard control, there is a deterioration in response of drive torque outputted when the accelerator pedal is depressed. Moreover, sometimes, fuel consumption or emissions deteriorate.
In the system taught by JP-A-2010-159815, drive torque is transmitted by a creep force, hence transmission torque decreases more compared to during a completely engaged state. Therefore, response of a drive power transmitted to a drive wheel worsens with respect to depressing of an accelerator pedal.
According to the teachings of JP-A-2005-313786, the lockup clutch is coupled between a crank shaft of the engine and the input shaft of the automatic transmission, hence it is possible to block inertia torque of the engine being transmitted to the automatic transmission. However, transmission of inertia torque occurring on a downstream side of the lockup clutch including the automatic transmission cannot be blocked.
Therefore, there is further room for improvement in improving drivability by overcoming a sluggish feeling given to a driver, while suppressing a shift shock resulting from, for example, a shifting operation by the driver to accelerate a vehicle.